The present invention relates to an ionization chamber making it possible to measure high energy gamma radiation. It more particularly makes it possible to measure gamma radiation with energy levels close to 6 MeV. Such gamma radiation is in particular emitted by nitrogen 16 induced, by nuclear reactions, in the water of a pressurized water reactor vessel. The measurement of this gamma radiation and consequently the nitrogen 16 quantity formed is one of the means used for obtaining information on the power of a reactor and for determining the speed and flow of the fluid circulating in the primary circuit of the reactor.
From a general standpoint, an ionization chamber comprises a tight enclosure filled with an ionizable gas, and one or more electrodes making it possible to create an electrical field within the enclosure. When nuclear radiation traverses the gas in the chamber, the latter is ionized. In the presence of the electrical field, the charges produced undergo, in the direction of the field, a pull which is superimposed on the thermal agitation. This pulling of the charges and particularly the ions formed makes it possible to induce, across the electrodes, a so-called ionization current, which is measured.
The ionization chamber representing a detector well suited to the measurement of gamma radiation can permit the measurement of the gamma radiation having a dose rate from between 1 and 100 rad/hour. Bearing in mind the environmental conditions and the fact that no human intervention is possible during the operation of the reactor, in order to carry out such gamma radiation measurements and particularly of radiation emitted by nitrogen 16, induced in the water of the vessel of a reactor, the ionization chamber must have a high sensitivity for the high energy gamma photons, a good resistance to high temperature, a good resistance to vibrations, a high pass band making it possible to measure fluctuations, considerable robustness and a long service life.
The ionization chambers at present used for such a measurement and which have a low thermal sensitivity have a low sensitivity to high energy gamma photons, which makes it necessary to use amplifiers, which are very sensitive to the temperature and to the ambient humidity.